Gas plasma etching devices are commonly used in the semiconductor fabrication and thin film industries to etch selected patterns on semiconductor wafer surfaces. Plasma etching devices generally comprise two large parallel plate electrodes contained within an evacuated chamber. The chamber is filled with a small amount of etching gases, usually chlorine and fluorine based compounds such as CF.sub.4 and CF.sub.6, and the wafer to be etched is placed on the lower electrode. A radio-frequency (RF) signal provided from a high-power signal generator is then applied between the parallel-plate electrodes to effect the etching of the wafer, as is well known to the etching art. Due to the high-power level of the RF signal, the Federal Communications Commission (FCC) has assigned a standard frequency value of 13.56 Megahertz (Mhz) for the signal generator so as to not interfere with other communication systems.
The etching rate of the plasma system and consistency of the final etched pattern depend heavily upon the efficient and consistent coupling of RF power from the signal generator to the plate electrodes of the plasma system. Efficient power coupling from the generator to the plate electrodes (the load) occurs when the load impedance of the plate electrodes has a value equal to the complex conjugate of the generator's characteristic impedance. The generator's characteristic impedance is the impedance seen looking into the output of the generator. (An impedance and its complex conjugate have the same resistance, or real part, and the same magnitude of reactance, or imaginary part. Their imaginary parts, however, have opposite signs.) High-power signal generators are generally designed to have a characteristic impedance having a resistance of 50 ohms and a reactance of zero ohms. The load impedance of the plate electrodes of the plasma system, however, is both capacitive and resistive (e.g., 5-j110 ohms) and is extremely mismatched from the impedance of the signal generator. Furthermore, the value of the load impedance changes during the etching process as the composition of the gases and the etching by-products in the chamber change.
This mismatch problem has generally been addressed in the prior art by placing a matching network in series between the signal generator and the mismatched load. The matching network usually comprises two or three reactive components, i.e., capacitors and inductors, arranged in either an L-, Pi-, or T-network. At a fixed frequency and with the proper component values, the matching network presents a 50-ohm load to the signal generator and a conjugate matched impedance to the load, i.e., the plate electrodes. As such, the matching network transfers substantially all of the power from the generator to the load. Little power is dissipated in the matching network since the matching network uses only reactive components.
As the load impedance varies, the values of the network's components must be changed to maintain the conjugate matches presented to the generator and load by the network. This process is often referred to as tuning the network or referred to as finding the tune point. In the above described matching networks, where they include three components, two of the components usually have variable values while the third has a fixed value. A servomotor is generally used to precisely position the mechanical control element of the variable component. Variable capacitors are generally selected as the variable-value components because they are easier to construct, are more mechanically reliable, and dissipate less power than variable inductors.
To guide in the adjustment of the variable components, a tune detector is generally placed in series between the signal generator and the matching network. It generally comprises a 50-ohm transmission line, which couples power through the detector without affecting the impedance matching process, and a plurality of sampling couplers. The samplers monitor the magnitude and phase relationships between the current and voltage in the 50-ohm transmission line. Under a tuned condition, the current and voltage waveforms are substantially in phase and the ratio of their amplitudes is substantially equal to 50 ohms. In general, the tune detector provides a signal indicative of the current-voltage phase relationship and a signal indicative of the amplitude-ratio relationship between the current and voltage waveforms. These signals are designed to each be near or at zero under a tuned condition. In some cases, the tune detector provides the phase and amplitude signals in the form of four or more basic signals which must be mathematically combined to provide the phase and amplitude signals. For example, see U.S. Pat. No. 4,679,007 issued to Reese, et al.
The automatic matching networks in the prior art generally include a controller of varying sophistication to interpret the phase and amplitude signals from the tune detector and to adjust the values of the variable components of the network. Most of these controllers comprise analog components (e.g., op-amp based comparators, summers, etc.) which implement two control loops, each loop for controlling one of the variable components in the network. The first control loop measures the amplitude signal and adjusts a first one of the variable components and the second control loop measures the phase signal and adjusts the second variable component. To adjust its corresponding variable component, each control loop measures the magnitude and sign of its corresponding signal (e.g., phase or amplitude) and adjusts the variable component so as to force the signal to zero, indicating a tuned 1 condition. In these systems, the sign of the corresponding signal determines whether to increase or decrease the value of the corresponding variable component.
Unfortunately, the prior art controllers suffer from a number of problems which impact the precision, responsiveness, and repeatability of the match between the high-power signal generator and the parallel plate electrode load. First, these controllers have a so-called "dead-band" designed into the control loop to ensure that the control loops do not "jitter" around the tune point. The jittering reduces the accuracy of the match, as the variable components are constantly changing, and it also wears out the motor drives used to mechanically adjust the variable components. The dead-band prevents unnecessary jittering but, unfortunately, causes a region of uncertainty around the tune point that is proportional to the size of the dead-band and, hence, reduces the precision of the tuning process.
A second problem arises in the prior art controllers in that the rate of change in the variable components (i.e., the rate of motor movement) is dependent on the characteristics of the control loops. Specifically, the control loops must be designed to be stable over a wide operating range. As a consequence, the region of highest instability places an upper limit on the rate of motor movement for each motor. Thus, the time required to reach a tuned condition is often slow.
A third problem in the prior art controllers arises because there are often false tuning points in the amplitude and phase signals provided by the tune detector. To prevent a false tune condition, the prior art controllers include additional detection and control circuitry which is designed to "kick" the control loops away from the false tune point. The additional circuitry is designed for worst case conditions and, as such, often provides too much of a "kick" away from the false tune point, thus increasing the tuning time.
The above drawbacks and constraints in the prior art systems have led to a degraded level of performance in gas-etching systems which is not commensurate with the precise and uniform etching tolerances currently needed in the semiconductor fabrication industry. The present invention is generally directed towards overcoming these drawbacks and constraints and improving the performance of automatic impedance matching systems in general and as used in plasma-etching systems.